1. I do not think there is any plugin available that still supports Photoshop 2.5 or includes preview scaling by default. Adding some image scaling code is not difficult, but I guess the plugin would also need to update the filter record and would need to use a self-allocated image buffer for the scaled down preview image. This again would need many internal changes. As GIMP is not very popular among Photoshop plugin users I would not want to invest my time to make the plugin preview work correctly in GIMP.

2. Actually we fixed the color dialog problem in our plugins quite some time ago.

3. I am not sure if this causes any bugs in my plugins.

How about you fixing these Problems with the GIMP plugin host? Would be the better solution than asking all plugin developers to change their plugins.